GS4104 is a neuraminidase inhibitor with a demonstrated anti-human influenza activity in vitro and in vivo. It has been administered to humans without producing serious side effects. This study is a multicenter, double-blind, randomized, placebo controlled study with two study groups. Adults from 14 years of age or greater, with chronic cardiac and/or respiratory disease will be randomly assigned to one of the two treatment groups:1) Ro 64-796 75mg bid; 2)matching placebo. Duration of treatment is 5 days but the entire study duration is 21 days. The study will begin when the first cases of influenza are noted in the Galveston area. The specific aims are: 1) To investigate the clinical efficacy of Ro 64-0796 inpatients with chronic cardiac and/or respiratory disease and a diagnosis of acute influenza; 2) To investigate the antiviral efficacy of Ro 64-0796 in this population; 3) To investigate the safety and tolerability of Ro 64-0796 in this population; 4) To investigate the impact of treatment of the acute disease on patients health and functional status and on the use of medical and non medical resources associated with influenza and its complications in this population. The research hypothesis is that Ro 64-0796 recipients will have a shorter and less severe course of influenza than the placebo recipients. The results obtained from this study may provide an alternative to influenza vaccination, amantadine and rimantadine in this population. An alternative to these drugs would be medically useful since some people cannot tolerate influenza vaccines due to allergy or other adverse effects. Influenza vaccine provides incomplete prophylaxis since the vaccine provides protection against influenza species from the previous year's strains of influenza, and the use of amantadine and rimantadine is limed by GI and CNS side effects as well as the development of viral resistance to these agents.